Field
The present invention relates generally to tunable impedances. In particular, the present invention relates to tunable impedances that can for example be used in wireless communications and more particularly in radio transceivers.
Description of the Related Technology
Many modern wireless standards such as HSPA+ (Evolved High-Speed Packet Access) and LTE (Long Term Evolution) require frequency division duplexing (FDD) operation. FDD is the process of transmitting and receiving with one wireless node at the same time, but at a frequency offset. Isolation between the transmitter and receiver is a critical issue to guarantee a reliable communication channel, since a typical receiver saturates in the presence of the interference generated by the transmitter (commonly referred as a self-interference). Surface acoustic wave (SAW)-based duplexers are typically used in modern cell phones to tackle the issue of self-interference by providing the necessary isolation. SAW components, however, have several issues. First of all, they cannot be integrated with a complementary metal-oxide semiconductor (CMOS) die, leading to a higher bill of material (BOM). Second, they are fixed frequency filters, and with the ever increasing amount of bands, this means more and more SAW duplexers are required.
A promising method has already been proposed to implement a tunable duplexer in CMOS, called electrical balance duplexing. Using this method, several fixed frequency SAW-based duplexers may be replaced by a single tunable duplexer, which decreases the bill of material. The concept of electrical balance duplexing is illustrated in FIG. 1. In electrical balance duplexing, the power amplifier (PA) 110 signal (i.e. at the transmitter (TX) port) is equally split across both the antenna impedance ZANT 120 and a so-called balance impedance network ZBAL 130. When the impedance of the balance impedance network ZBAL 130 is equal to the antenna impedance ZANT 120, the TX signal is common-mode to the differential balun 140, and ideally no signal couples to the low-noise amplifier (LNA) 150 side, i.e. no current is induced in the secondary winding of the transformer on the low-noise amplifier (LNA)-side 144 (i.e. the receiver (RX) 150 port). This is called electrical balance. In this way, the low-noise amplifier 150 from the RX is isolated from the TX, so that no self-interference problems occur.
Ideally, both the balun 140 and the balance network ZBAL 130 are integrated together with the transceiver to save on the bill-of-materials. Modern software-defined radio (SDR) implementations are implemented in digital CMOS so that the baseband processor are integrated on the same die as the analog front-ends.
Several implementation problems of the antenna impedance balancing network (ZBAL in FIG. 1) limit practical implementations of electrical balance duplexing. First, the impedance of the balance impedance network ZBAL 130 has to be tunable to cover the required antenna impedance ZANT 120 across frequency. Tunable components implemented as for example a network of switched passive components (inductors, capacitors and resistors) may provide a tunable passive impedance, which depends on the passives that are switched on.
Second, the tunable components need to be highly linear in order not to degrade achieved transmitter chain linearity performance, even upon high PA power. At high PA power levels, a certain adjacent channel leakage ratio (ACLR) performance is achieved by the transmitter chain, to adhere to the transmission mask as defined by the standard. When a non-linear component is added between the PA 110 and the antenna 120 (e.g. a duplexer with non-linear switches), this ACLR performance should not be degraded. A simple measure of linearity performance of any RF component is the input-referred 3rd order intermodulation extrapolation point (IIP3). A typical example shows that the required IIP3 should be as high as +53 dBm at 30 dBm output power while achieving a TX chain performance of −40 dBc ACLR.
Third, the switchable components in the balance impedance network ZBAL 130 have to withstand the high voltage swings of the PA 110 (which may have peak RMS levels as high as about 10 to 20V). In digital CMOS, the main method of switching passive impedance components is with thin-oxide transistors. Thin-oxide MOS transistors have low breakdown voltages (up to about 3V) and the PA signal severely limits lifetime expectancy of such implementations, if it would not cause immediate breakdown. If, as described by Minsik Ahn et al. In “A 1.8 GHz 33-dBm P 0.1-dB CMOS T/R Switch Using Stacked FETs With Feed-Forward Capacitors in a Floated Well Structure”, IEEE Transactions on Microwave Theory and Techniques, Vol. 57, No. 11, November 2009, stacked-FETs with feed-forward capacitors in a floating well structure are used as switches, high-voltages could potentially be sustained in CMOS.
However, in this solution, multiple devices are used to be able to withstand higher voltages. For some implementations, sufficient linearity may not be achieved. Unlike generic CMOS switches, these stacked-FET switches may suffer from additional parasitics, possible impacting tuning range performance. Finally, implementation may not be straight-forward, since models of such devices may not be readily available.